The Metropolitan Police is to keep its water cannon so its officers can be trained for potential deployment to Northern Ireland.

Although the Home Secretary has refused to allow the use of water cannon in England and Wales - a year after three cannon vehicles were bought by the Metropolitan Police - the Met has said it will not be disposing of them as its officers occasionally travel to Northern Ireland to provide mutual aid to the PSNI.

"This can require them to deploy alongside water cannon. We will be retaining our devices to ensure our officers are properly trained for this eventuality," the Met said.

"This will also give us access to equipment that is already used elsewhere in the UK, but only on specific political approval, as is appropriate and one that is likely to be granted only in the most extreme of circumstances," the force added.

The PSNI has the ability to request assistance from 300 police officers from forces across the UK in cases of serious and sustained civil disorder. The officers could be deployed within 48 hours following a request for help by the PSNI Chief Constable.

No requests have been made for mutual aid support so far this year. Officers from other UK police services were called in to help PSNI officers in July 2013 following several nights of serious rioting after an Orange Order parade was stopped from returning along part of the Crumlin Road in north Belfast.

The mutual aid officers are trained alongside PSNI officers. As water cannon is not used anywhere else in the UK, mutual aid officers also need specialist training should they be deployed alongside it.

Water cannon were deployed most recently in north Belfast on Monday against loyalist demonstrators during the annual Twelfth of July celebrations.

The Met have argued the case for water cannon in London and three were bought from the German police at a cost of £328,883.

However, on Wednesday Home Secretary Theresa May refused to allow their use in England and Wales.